1. Field of the Invention
The present invention relates to the technology of authentication systems, authentication methods, server devices, and client devices.
2. Description of the Related Art
Various systems have been proposed, in which server devices for providing services authenticate client devices for receiving the provided services. For example, merely causing a client device to simply input authentication information, such as an ID and a password, imposes burdensome set-up work on a user.
Thus, in International Publication Pamphlet No. WO 2011/080867, as a method of automatically setting authentication information, a method is described, in which an optical signal resulting from modulation of authentication information is transmitted from a light emitting diode to a client device. Thereby, the security is able to be improved, by controlling not to provide any service unless a distance between a server device and the client device is equal to or less than a certain distance.
In a system, where authentication information is automatically set, if a malicious user is able to receive the authentication information, a server device will be automatically abused, and thus, a means to improve the security more while keeping the convenience of the automatic authentication is desired.
For example, according to the method in International Publication Pamphlet No. WO 2011/080867, if a malicious user is able to receive at a certain moment, even at one location, the optical signal resulting from modulation of the authentication information, there is a risk that the authentication information may be abused.